The invention relates to surface acoustic wave (SAW) delay lines, and particularly provides a simple tapped meander delay line.
Rectilinear SAW delay lines are known, for example as shown at 2 in FIG. 1, "Surface Acoustic Wave Multistrip Components and Their Applications", Marshall et al, IEEE Transactions On Sonics and Ultrasonics, April 1983, pages 134-144, at FIG. 4. A SAW is propagated along path 4 on piezoelectric substrate 6. A plurality of multistrip couplers such as 8-11, each of which is an array of parallel metallic strips deposited on the piezoelectric substrate, transfer acoustic power from one acoustic track or path such as 4 to another such as 5. A multifinger RF interdigital transducer 12 is formed by an array of alternately interconnected parallel metallic strips deposited on the piezoelectric substrate, and transduces an RF signal to an acoustic signal or SAW. The leftward propagation of the SAW from transducer 12 is terminated by absorber 14. The rightward propagation of the SAW from transducer 12 is coupled by multistrip coupler 8 such that a portion of the SAW continues along path 4 and a portion is transferred to upper parallel path 5, the leftward propagation of which is terminated by the upper portion of absorber 14, and the rightward propagation of which is converted to an RF signal by multifinger RF interdigital transducer 16. Absorber 18 terminates rightward SAW propagation from transducer 16. Like operation is provided by multifinger RF interdigital transducer 20 and absorber 22, RF transducer 24 and absorber 26 and so on. This is a multitapped delay line with different length delays of the RF signals from transducers 16, 20, 24 and so on.
The total delay time in the structure of FIG. 1 is limited by the length of the rectilinear SAW track. Overcoming this limitation is another known SAW delay line, as shown in FIG. 2, which is folded back in serpentine manner to provide a meander type delay line, "Surface Acoustic Waves for RF Memory", Wang et al, Polytechnic Institute of New York, December 1982. Multifinger RF interdigital transducers 30 and 32 provide input and output ports for an RF signal which is transduced to a SAW propagating back and forth between reflecting multistrip couplers 34, 36, 38 and so on. This meander type delay line is limited to a single delay time between input 30 and output 32, without plural selective delay times as in the tapped delay line of FIG. 1.
The present invention combines the advantages of the tapped delay line of FIG. 1 and the meander type delay line of FIG. 2. The present invention provides a tapped meander delay line.
Much work has been done on multistrip components to perform various functions, Marshall et al, supra. As shown in FIG. 3, a SAW from RF transducer 40 can have its propagation path altered by multistrip coupler 42, for example a 90.degree. change in direction. The U-shaped multistrip couplers such as 44 in FIG. 4 have been used as reflecting elements for returning the SAW back to RF transducers such as 46 and 48.
U-shaped reflectors such as 44 have been combined with straight multistrip couplers to provide a reflecting track changer 50, FIG. 5, which may be used as the reflectors such as 34 in FIG. 2. Reflecting track changer 50 includes a 3 dB straight multistrip coupler 52 and a pair of U-shaped multistrip couplers 54 and 56 facing each other at the ends of the arms of the U's.
As noted on page 138 in the above noted Marshall et al article, a unidirectional transducer may be constructed by combining a multifinger RF interdigital transducer such as 60, FIG. 6, with a U-shaped multistrip coupler 62. Interdigital transducer 60 is provided with an odd number of fingers and is placed within the arms 62a and 62b of the U so that the center of the central finger 60a is offset, relative to the center line of U 62, by 1/8 wavelength towards the desired output port. This in turn eliminates the need for absorbers to terminate a SAW otherwise propagating in the other direction. The structure is inherently frequency band limited to the 1/8 wavelength offset.
In another application of angled couplers, the U-shaped structure can be formed into a J, as shown at 70, FIG. 7, by extending one arm of the U at 72 into an adjacent track 74. In conjunction with a bidirectional RF transducer 76, the J forms a unidirectional transducer and tap. Without transducer 76, it acts as a reflector.